Design guidelines for Direct Metal Laser Sintering (DMLS)

Copyright © Crucible Design Ltd. 2015

Design guidelines for Direct Metal Laser Sintering (DMLS) Introduction

Direct Metal Laser Sintering (DMLS) is an

additive manufacturing technology that is

based on melting layers of metal powder

using a powerful carbon dioxide laser.

The fact that layer-based production

processes do not require tooling has led to

the assumption that these technologies are

free of rules that govern design for

manufacture, like draft angles and

undercuts.

Whilst DMLS does not have to observe

conventional design rules, there are others

that do apply to the process, particularly if

reducing cost and waste is a consideration.

This presentation looks at these rules and

how they affect the design of parts built

using the DMLS process.

Design guidelines for Direct Metal Laser Sintering (DMLS) The benefits of DMLS

The primary benefits of DMLS are:

■ The ability to create internal as well as external complexity

■ The potential to build multiple parts in one process, maintaining joints, hinges, etc

■ The absence of any tooling, allowing complex shapes and geometry to be produced

■ The ability to vary wall sections to achieve optimum strength

Design guidelines for Direct Metal Laser Sintering (DMLS) The environmental impact of DMLS

The DMLS process requires a considerable amount of

energy, particularly if the costs of primary material

extraction and processing are taken into consideration.

However, considerable energy savings are possible if the

process is used to create parts that reduce CO2 emissions

as a result of their long term use.

Most of these applications involve reduced weight. Any part

that accelerates, for example, will save energy if its weight

is reduced. This is particularly true of any part that is used in

aerospace applications, especially if the part is used many

times. Another area is improved thermal efficiency,

particularly of parts like heat sinks, etc.

The way DMLS parts are designed can also have a

significant effect on their energy use, particularly in terms of

the way the part is built within the process, the amount of

post-machining that is required and how much waste is

generated. These issues will all be examined in the

following sections.

Design guidelines for Direct Metal Laser Sintering (DMLS) The DMLS process

1. A layer of powder (approx 0.02mm to 0.08mm thick) is deposited

on the build platform by a recoating blade. The bottom layer of the

part is then created by the laser, which locally melts the powder.

2. The roller then deposits another layer of powder

as the build platform lowers one layer and the

powder reservoir is raised by the same amount.

3. The laser then fuses the

second layer to the first.. and so

on...

4. As the layers build, the level in the build

chamber goes down and the powder reservoir

base rises.

5. When the final layer of the part has

been built, the powder is removed.....

...revealing the part attached to the

build platform.

Design guidelines for Direct Metal Laser Sintering (DMLS) The materials

Design guidelines for Direct Metal Laser Sintering (DMLS) The build platform

Build platform is made of steel plate. One

standard size for the base is 250mm x

250mm.

Parts are built vertically up from the base. Multiple

layers of parts are only possible if they can be

stacked on top of each other and cut apart after the

build.

Build platforms are typically 250mm x

250mm at the base and either 215mm or

315mm high.

Before the parts are removed from the base, the whole platform is placed in a

furnace for several hours to relieve mechanical stresses. The parts are then

removed from the base by EDM (Electrical Discharge Machining) - also known

as 'wire cutting'. The path can help define the geometry of the finished part.

Material left on the base (usually only a

few millimetres) is removed by

machining....

....and the machined base is now

ready for another build.

multiple parts stacked

cutting path

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – building layers

The most straightforward geometry to build in DMLS is a vertical 'extruded' form from

the build platform, where each layer builds on the geometry directly below it.

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – angled surfaces

The powder in the build chamber

does not provide any support to

the part as it builds, so any angled

surfaces will ideally be self-

supporting.

If the angle is too acute, the surface will

need a supporting structure built in as part

of the model. This supporting structure will

then need to be removed by machining or

wire cutting, increasing energy use.

The minimum angles that will be self

supporting are approximately:

- Stainless steels: 30 degrees

- Inconels: 45 degrees

- Titanium: 20-30 degrees

- Aluminium: 45 degrees

- Cobalt Chrome: 30 degrees

If the angle is near the point where it

needs supports, the downward facing

surface will become rough and may

require considerable post-finishing.

Small holes can be accommodated

easily. Holes of less than 6mm

diameter are ideal.

Larger circular holes will result

in a roughened surface at the

top which may need post-

machining.

Large holes will require support structures to

be added in the centre to prevent the part

collapsing or becoming distorted during the

build process. These supports will need to

be removed by wire cutting or machining.

If the hole has an angled or arched

upper area it will probably not require

any supports. This is one of the features

of DMLS that can have a significant

impact on the design process.

supporting structurerough surface

rough surface

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – downward facing surfaces

supports

Any downward facing surface will require support.

Support structures will need to be removed by

wire cutting or machining, which will increase the

energy and waste involved in the process.

The most simple support structure will fill the hole

that creates the downward facing surface. This

can be removed by wire cutting or machining.

An offset support structure can be used that will be easier to remove.

In this case, the base of the support will be cut when the part is

removed from the base by wire cutting, leaving one edge to be cut in

order to remove the rest of the support.

An alternative to this approach will be to turn the part through 45 degrees to

make all the surfaces angled and remove the need for supports. Orientation is

a major issue in finding the most efficient build method - please see item 3 in

Other Issues for more details on the limits and possible pitfalls of using angled

edges like the ones shown above...

If the top surface of the hole can be

made of a series of angles (which are

self supporting) the supports can be

minimised to the base of each angled

surface.

If the hole is simply for weight reduction or cooling, for

example, it can be modified as a series of semi-

circular topped slots which will not require supports.

However, the 'pillars' between the holes need to be

self-supporting.

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – direction of build and cross sections

As the re-coater blade passes over the part, depositing

another layer of powder, it can touch the layer below,

sometimes with force. The orientation of the part is,

therefore, important. The ideal geometry is a circular

profile which provides a smooth lead in for the blade, and

a stable cross section as it builds.

An open 'U' or similar shape is also ideal,

as the lead in for the blade is again

rounded, and the basic profile will be strong

as it builds, resisting the force of the

recoating blade.

The 'worst case' geometry would be a thin

section parallel to the re-coater blade. The blade

will tend to 'bounce' off the parallel wall, and the

section itself will not resist the force of the blade

as it builds.

Any flat surfaces need to be at least 5 degrees

from parallel with the blade to allow the blade

to touch the part at a point, not a face.

In addition to touching the part at an angle, it

helps if the geometry is inherently stiff, which will

resist bending forces as the re-coater blade

passes over the part.

Long, thinner parts with rounded ends will build well, as

they also provide a smooth lead in for the blade and are

inherently stiff. However, all these issues need to be

considered in parallel with the other limits (build angles,

etc) mentioned elsewhere in this section.

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – part strength during the build process

part will be very

weak at this point in

the build process

As the re-coater blade passes over the part, more

force will be applied to the geometry as it gets taller.

As a rule of thumb, the ratio between the section and

the height should be no more than 8:1.

The exact proportions will always depend on the specific

geometry, but if the section gets too high, there is a danger

that the re-coater blade will bend the part, and possibly

damage itself in the process, terminating the build sequence.

To prevent these problems, vertical sections need

to be bridged at certain points. The best method

of achieving this will be to use 'arches' to avoid

the creation of downward facing flat surfaces.

Even a part that will be strong when it is

finished may need some support during the

build process. This triangular section will be

very weak as the build gets close to the apex.

This kind of structure may need a

simple support structure up the

middle to provide some rigidity

before the part is completed.

If the reason for the open structure is simply

weight reduction, it may be easier to perforate it

with holes (ideally less than 6mm in dia) that will

reduce weight, but not require any supports.

Design guidelines for Direct Metal Laser Sintering (DMLS) Design basics – other issues

1. Avoid sharp edges. Very sharp edges cannot

be built in DMLS, and it is better to design parts

with minimum radii of approximately 0.5mm.

2. Avoid thick sections. The heat build up when creating very

large horizontal sections can affect the build geometry,

particularly when using titanium. A better approach is to angle

the part to minimise the horizontal section at any one time.

3. Avoid angles facing into the re-coater blade.

Angled parts that lean into the path of the re-

coater blade may cause the blade to collide

with the part and terminate the build.

4. Avoid sharp edges. Sharp corners can act as

'stress raisers' in DMLS in the same way as they

can in most processes. Always try to use radii on

corners instead of sharp edges.

5. Use the wire cut removal path. The path

used to wire cut the part from the base can be

used as an integral part of the component

design, rather than simply as a straight cut.

6. Build multiple parts. The nature of the DMLS

process allows for multiple parts to be built 'in

situ'. This can save considerable time and

assembly cost for appropriate geometry.

Design guidelines for Direct Metal Laser Sintering (DMLS) Supports – what do they do?

Supports are a 'necessary evil' in the DMLS

process. Good design practice will minimise them,

as they use a lot of energy - both in their

construction and removal - but they also fulfil a

number of vital functions within the process:

1. They support the newly melted surface,

particularly on downward facing surfaces and

shallow angles.

2. They can prevent the new geometry from

deforming.

3. They dissipate heat away from the newly formed

geometry, and

4. They provide temporary support for geometry

that will be strong when complete, but that is

weak during the build process. (see 'part

strength during the build process').

The ideal situation is to design a part that requires

no supports at all (see images on the right for

theoretical worst and best case scenarios).The

reality is that it is rarely possible to design parts that

require no supports at all, but minimising them will

save time, energy, and money.

Large amount of support structure that

needs to be built (and then removed) to

support downward facing surfaces during

the build process

Geometry changed to simple curve

that can be built without supports

Design guidelines for Direct Metal Laser Sintering (DMLS) Supports – types of support

1. Simple fill in. The most simple form of support is to fill in the area that needs

support, and then cut this out when the build is complete by wire cutting or

machining. If the support area is to be removed with wire cutting, a small hole

needs to be placed in the support area to allow the wire to be located.

2. Offset supports. Offset supports require less

machining. They rise vertically and then angle in to

support specific surfaces. The base of the support is

usually removed with the wire cut removal of the part,

requiring only the supported surface to be machined.

All support structures are

formed from fine lattices, to

minimise energy

consumption and build time

3. Overhanging surfaces. Horizontal overhanging

surfaces can be supported from the base, although

this will require a considerable amount of material and

energy. A better solution is to 'buttress' the surface

from the main geometry at an angle. Better still,

design the support into the geometry and remove the

need for any additional work.

support

from base

‘buttress’

support

4. Supports for curved surfaces. Sometimes, it is necessary to

support a downward facing curved surface to prevent the

geometry failing or a very rough surface being formed. In this

case, a support structure is formed under the part which is then

removed by wire cutting or machining when the part is removed

from the base.

hole for

wire cutting

supports

removed

Design guidelines for Direct Metal Laser Sintering (DMLS) Heat treatment, post machining and finishing

Heat treatment

The build platform needs to be heat

treated before the parts and

supports are removed. There are

two reasons for this:

■ Age hardening of the

parts, and

■ Stress relieving prior to

removing the parts.

There are three methods commonly

used:

■ Electrical furnace

(above)

■ Vacuum furnace, and

■ Hot Isostatic Pressing

(HIPing)

Removal of supports

Supports can be very light tubular

forms, like the ones shown above

supporting dental copings, or heavy

sections supporting large downward

facing surfaces. They are usually

removed either by wire cutting - where

there is clear path for the wire - or

machined off, either by CNC

equipment or manually.

The best supports are those that can

be removed easily by the same wire

cutting process that is used for

removing the parts themselves.

Post finishing operations

In addition to removing the supports, DMLS parts

usually require some post finishing. This can

include any of the following:

■ CNC machining (to achieve the required

tolerances)

■ Blasting (to improve surface finish and relieve

stresses):

■ Shot peening: Steel & ZrO2 media

■ Ti: Wet blasting with Al2O3 media

■ Polishing (to achieve a good cosmetic finish):

■ Manual

■ Automatic (Microtek-MMP Process)

To find out more about this process,

visit www.firstsurface.co.uk.

Design guidelines for Direct Metal Laser Sintering (DMLS) Other considerations - the impact of part stresses

The very high temperatures

involved in the DMLS process can

cause significant stresses to build

up in parts as they build. These

stresses can result in:

■ Delamination of the

layers

■ Cracks in the part

■ Distortion and

■ Warpage during post

finishing

These problems can be minimised

or avoided by shot peening the

parts to relieve stresses and/or

heat treating prior to removing the

parts from the build platform.

Strong support structures can also

minimise the build up of stresses in

the part.

Design guidelines for Direct Metal Laser Sintering (DMLS) Other considerations - forming threads in DMLS parts

Threads can be formed

directly into parts,

depending on the size of

the thread and the

orientation. Threaded

areas should always be

vertical, and ideally have

sufficient clearance

around the thread to allow

a tap or die to be used to

ensure that it is clean.

Smaller threaded areas

should be left off the CAD

file, and post-machined

(Drilled and tapped or

thread milled).

Design guidelines for Direct Metal Laser Sintering (DMLS) Other considerations – wall thicknesses

Wall thicknesses are somewhat

material dependent, but as a rule

of thumb, wall sections should

not fall below 1mm.Very thin wall

sections - or placing a thin

section against a thick section -

may result in significant distortion

due to the very high temperatures

involved in the process.

Fine detail is possible, however,

particularly in the vertical plane.

The illustration on the right shows

a section of a pipe with a wall

section of 0.9mm with a hole

running through it of 0.4mm

diameter.

Design guidelines for Direct Metal Laser Sintering (DMLS) Case study – bicycle pedal

A conventional 'rat trap' bicycle pedal (left) has a large number of surfaces. If it is built in the

horizontal plane, the large number of downward facing surfaces will require a significant

amount of support (right). A large number of these can be offset, which will reduce the

removal time, but building the part would require a considerable amount of energy.

If the geometry is modified to reduce the number of downward

facing surfaces (mainly by putting in a number of 45 degree

angled surfaces) the amount of supports needed is reduced

significantly (right).

However, by changing the orientation of the

part to vertical, the number of supports

needed is dramatically reduced.

This vertical orientation, combined with design changes to the pedal, would

allow designs to be produced that require no supports at all.

Design guidelines for Direct Metal Laser Sintering (DMLS) Conclusions and critical issues

■ The design rules for DMLS are based on

the efficiency of the build process.

■ The design needs to consider the

construction of the part layers throughout

the process, particularly those stages

when the part will be inherently weak.

■ Part orientation is critical – particularly in

relation to the re-coater blade and the

minimisation of supports.

■ Consider the wire erosion process that

removes the part from the build platform

as part of the design – it can form critical

geometry and reduce post-machining

time and cost.

■ Get creative. Design around these

limitations using the strengths of the

process – the ability to build layers on top

of each other that ignore conventional

manufacturing geometry.

Design guidelines for Direct Metal Laser Sintering (DMLS)